The study of magnetic core components used in electrical/magnetic energy conversion devices such as generators and transformers requires analysis of several physical and electromagnetic properties for the core component. Two key characteristics of an iron core component are its magnetic permeability and core loss characteristics. The magnetic permeability of a material is an indication of its ability to become magnetized, or its ability to carry a magnetic flux. Permeability is defined as the ratio of the induced magnetic flux to the magnetizing force or field intensity. When a magnetic material is exposed to a rapidly varying field, a resultant energy loss in the core occurs. The core losses are commonly divided into two categories: hysteresis and eddy current losses. The hysteresis loss is brought about by the necessary expenditure of energy to overcome the retained magnetic forces within the iron core component. The eddy current loss is brought about by the production of electric currents in the iron core component due to the changing flux caused by alternating current (AC) conditions.
Early magnetic core components were made from laminated sheet steel, however, these components were unsatisfactory due to large core losses at higher frequencies and due to manufacturing difficulties. Application of these lamination-based cores is also limited by the necessity to carry magnetic flux only in the plane of the sheet in order to avoid excessive eddy current losses. Sintered metal powders have been used to replace the laminated steel as the material for the magnetic core component, but these sintered parts also have high core losses and are restricted primarily to direct current (DC) operations.
Research in the technology of magnetic core components has recently been centered around the use of unsintered iron-based powders which contain various coatings upon the iron powder particles. This research has strived to develop iron powder compositions which enhance certain physical and magnetic properties without detrimentally affecting other properties. Desired properties include a high permeability through an extended frequency range, high pressed strength, low core losses, and suitability for compression molding techniques.
When molding a core component for AC power applications, it is generally required that the iron particles have an electrically insulating coating to decrease core losses. The use of a plastic coating (see Yamaguchi U.S. Pat. No. 3,935,340) and the use of doubly-coated iron particles (see Soileau et al. U.S. Pat. No. 4,601,765.) have been employed to insulate the iron particles and therefore reduce eddy current losses. However, these powder compositions require a high level of binder, resulting in decreased density of the pressed core part and, consequently, a decrease in permeability. Moreover, if the use of such iron powder mixtures in a compression molding operation requires heating the die, high stripping and sliding ejection pressures are generated in the absence of an appropriate lubricant. This results in increased die wear and scoring of the pressed component. The use of conventional die wall lubricants such as zinc stearate, which were effective at room temperature compression molding, are not useful at the higher temperature compression conditions required to generate resin flow necessary for the molding of coated powder compositions.
Ochiai et al U.S. Pat. No. 4,927,473 discloses an iron-based powder composition whose particles are covered with an insulating layer of an inorganic powder such as boron nitride. These coated particles are used to form a magnetic core by compression molding techniques. The coated iron particles do not contain any outer coating or second coating of a thermoplastic resin, the absence of which, it has now been found, leads to lower core strength.
A need therefore exists in the art for an iron powder composition which is characterized by properties which include a high permeability through an extended frequency range, a relatively high pressed strength, reduced core losses, and reduced stripping and sliding ejection pressures when molded.